1. Field of the Invention
The present invention relates to a closure element for a butterfly valve having a valve seat perpendicular to the axis of the valve.
It is applied more particularly, but not exclusively, to butterfly valves in which the seat, of circular configuration, is situated in a plane perpendicular to the axis of the valve and is optionally positioned off-center relative to the drive axis of the butterfly gate.
2. Discussion of the Prior Art
It is known that in such valves, closure is in general obtained, starting from the open position of the valve, by a rotation of the butterfly gate from an angle substantially equal to 90.degree., at the end of which the edge of the butterfly gate contacts and compresses the valve seat, thus closing the valve.
In a more precise way, the valve seat which can consist, for example, of a flexible metallic joint or an elastomeric joint, assures on the one hand, a completely closed valve, referred to as the static condition, and, on the other hand, an open valve position referred to as the dynamic condition.
In the presently known butterfly valves, in which the butterfly gate is offset or off-center with respect to two axes, there is presented a peripheral edge surface of spherical form coming to rest on a circular joint, compression being effected by a displacement of the spherical surface encompassing the surface of the peripheral edge of the butterfly gate, herein referred to subsequently as the spherical surface, resulting in the movement of the spherical surface into the circular joint. One will note concerning this that in general the spherical surface of the butterfly gate is centered in the plane passing through the axis of the shafts of the butterfly gate which is parallel to the plane of the butterfly gate.
Thus, in such valves, the relative butterfly gate seat displacement is obtained by positioning the axis of the shafts and the plane of symmetry of the butterfly in off-center positions. The angle formed by the displacement vector of the butterfly gate and the tangent to the aforementioned spherical surface, is very small, which results in substantial friction between the seat and the butterfly. As a consequence, the restraining forces resulting from the action of the butterfly gate on the seat produce a considerable torque relative to the axis of rotation, which gives rise to significant torque forces both in opening and closing the valve.